You should address why s/hes in the water to begin with. Some common reasons are that it may have mites, it may be too hot in the enclosure, that the hide isn't snug enough and they are seeking comfort in the dish, or that there isn't enough humidity in the enclosure.

After you rule everything out, take the snake to the vet and make sure there is nothing medically wrong with her/him.

Sometimes they just do it, we don't really know why but it's really not normal and they CAN get scale rot from being too moist all the time.